It is believed that presbyopia occurs as a person ages when the lens of eye loses its elasticity, eventually resulting in the eye losing the ability to focus at near distances (when distance vision is corrected), such as the normal reading distance, and in some cases at intermediate distances. Presbyopic persons (presbyopes) complain of difficulty performing close tasks. To compensate for presbyopia, ophthalmic lenses are required to be more positively powered or less negatively powered than the distance correction. Some presbyopic persons have both near vision and distance vision defects, requiring simultaneous or alternating vision lenses, instead of single vision lenses, to properly correct their vision.
A typical single vision contact lens has a real or virtual focus, which is the point on which parallel rays of light focus when the lens is placed perpendicular to the parallel rays, and an optical axis, which is an imaginary line drawn from the focus to the center of the lens. A posterior surface fits against the cornea and an opposite anterior surface has a vision surface that focuses light to correct the eye's vision. In the case of a typical spherical optical zone surface lens, the vision surface has a single radius of curvature that is the distance from any point on the vision surface to a point on the optical axis referred to as the center of curvature.
Simultaneous vision refers to the class of bifocal or multifocal contact lenses in which optical power for distance vision and near vision are positioned simultaneously within the pupil area of a user's eye. The conventional clinical understanding for simultaneous vision is that a bifocal lens projects both distance and near images simultaneously onto the retina. Depending on the viewing distance of the object of regard, one of the images is in focus, and the other image is out of focus. It is believed that the brain is able to discriminate between the irrelevant out-of-focus image and to process only the relevant in-focus image. Therefore, whether an object is at distance or near locations, the lens is still able to provide levels of vision that may be acceptable to many patients.
Simultaneous vision contact lenses are generally composed of, within the pupil area of the eye, two or more concentric annular zones which alternately provide the distance and near power, or a multifocal zone having an aspheric surface which provides a continuous gradient of optical power over a selected range of powers. The visual performance of the simultaneous vision lens design is limited by its dependence on pupil size. Moreover, with all simultaneous vision lenses a partially degraded image of an object is projected onto the retina. The consequence of this image degradation is a reduction in visual actuity and/or contrast sensitivity (less signal, more noise), and the quality of the degraded image may or may not be acceptable to the patient. The clinical effects of this degradation may be measured objectively in terms of reduced visual acuity and contrast sensitivity. The subjective effects of the degradation are perceived by the patient in various ways which are collectively referred to as subjective blur. Therefore, when wearing a simultaneous vision lens, the patient may not be selecting between separate distance and near images. Rather, in the presence of subjective blur the patient may be attempting to function with the reduced level of spatial information content that is provided by a degraded image.
Alternating vision refers to the class of segmented (or translating) bifocal contact lenses in which the lens is divided into two optical zones. Typically the superior (or upper) zone is for distance vision correction, whereas the lower zone is for near vision correction. The distance portion (upper zone) subtends the pupil of the eye in primary gaze, while in downward gaze, the add power or near portion (lower zone) of the lens subtends the pupil. Effective use of an alternating vision lens requires vertical translation of the optical zones across the pupil when the eye changes from primary gaze to a downward gaze. In such a situation, the lens must move such that the pupil is predominately subtended by the distance zone for primary gaze and predominately subtended by the near zone for down-gaze. The visual performance of the alternating vision lens design is not significantly limited by its dependence on pupil size.
While there are many designs for hard bifocal translating contact lenses, controlled rotational stability and translation are difficult to achieve in soft contact lenses designed to translate across the surface of the eye when the visual direction of the eye changes from primary gaze (distance vision) to down gaze (near vision). There are several lens designs reported for soft bifocal translating lenses (see, for example, U.S. Pat. Nos. 4,618,277, 5,071,244, 5,371,976, 6,109,749). However, there are some disadvantages associated with those designs (for hard and soft translating bifocal contact lenses) in the prior art. First, bifocal translating contact lenses in the prior art are not comfortable to be worn. Second, the images from different optical zones in bifocal translating contact lenses in the prior art have lateral separation, causing ghost images to the wearer. Specifically, in a bifocal, segmented optical zone contact lens, the images, on the retina, from both an upper and lower optical zones can not be laterally coincident to minimize or eliminate ghost images. Therefore, bifocal translating contact lenses in the prior art can not provide clear or optimized vision to wearers.
Additionally, with the increasing use of computers, more and more people utilize a computer at work and at home. It is desirable that a translating contact lens could have one or more intermediate vision zones in addition to the near and distant vision surfaces. For example, it is desirable that the optical zone of a translating contact lens could have a continuous power profile or discrete optical sub-zones blend together or combinations thereof. Such a translating contact lens may have to have an ability to control the rotational stability and translation magnitude on the eye in a manner that the pupil can be subtended by one of near vision surface, intermediate vision surfaces, and distant vision surface.
Therefore, there is a need for a new type of multi-focal contact lenses, which are capable of providing distance, intermediate, and near vision corrections and have a visual performance that may not be limited by their dependence on pupil size.